1. Field of Invention
The subject invention is generally related to electronic data transmission over common carrier lines such as with wired or wireless telephone lines and is specifically directed to a method for recovering data lost due to noise and other interference signals occurring during transmission.
2. Discussion of the Prior Art
In recent years, the electronic creation and/or transmission and reception of data has exploded. The use of digital encoding, compression, transmission, decompression and decoding has made it possible to almost instantaneously transmit both visual and audio information to almost any location in the world. Examples of such applications include facsimile transmission of written or printed documents. For example, "fax" machines are now quite common, where an original document can be scanned and digitized, with the information representing the document being digitally transmitted of a phone line to a compatible receiving "fax" machine at any location, simply by dialing the telephone address of the receiving machine.
With the proliferation of electronic transmission of data, and the increasing speed of transmission, the potential for error in transmission due to noise and other interference has become more of an issue. This is particularly true when large volumes of data are transmitted during off hours when there often is no operator available to detect and intercept problem transmissions. For example, many large organizations routinely transmit documents at night. If such a transmission were interrupted by noise in the carrier lines, the transmission may be lost and no knowledge of the error would be found until the next morning. Another area of increasing concern is the transmission of data such as, by way of example, facsimile transmission, via cellular telephone systems, radio circuits and other non-hardwire transmission systems. There is inherent noise when switching cells as well as natural and man made interference, potentially causing a complete loss of transmission.
Typically, all electronic transmission schemes use an initializing scheme which is a "sign on" or initializing sequence to identify the sender and protocol. This usually begins with a sync and/or beginning of message signal (B-O-M) which identifies the beginning of a data transmission. This is generally followed by a header, after which the data flow begins. The data is often sent in blocks with FEC and CRC codes. For example, the header includes a cyclical redundancy check (CRC) which is repeated periodically throughout the message. At the end of the transmission, and end of message (E-O-M) signal is generated, and the transmission is completed. Typical protocols use CRC's to detect errors and FEC's to correct errors (on a limited basis) without retransmission, and with higher level protocol stacks to direct retransmission of detected lost data. Both use of FEC coding and retransmission require additional time to accomplish transmission. It would be far more desirable to minimize loss of data rather than retransmit it, particularly with real time correction or reconstruction. If the sync or B-O-M signal is missed, the entire data stream is lost. That is, when a transmission is initiated, a B-O-M signal is at the front end or beginning of the message. If the B-O-M signal is lost or missed, the data cannot be interrupted, and the entire transmission is lost. Once lost or interrupted, the transmission has to be resent with an identifying B-O-M signal. The only indication of an error is the failure to receive a reverse channel acknowledgment, if used, indicating that the transmission was interrupted prior to completion of the transmission.
A prior art and typical prior art transmission sequence are shown in FIG. 1. Typically, when an incoming bit stream is detected by a receiving system, as indicated at 10, the system looks for a sync signal (at 12). If a sync signal is detected, the system looks for a B-O-M signal (at 14), and if present, the data is collected (at 16). This process continues until an E-O-M signal is received (at 18), indicating completion of the transmission (at 20).
If either a sync signal or a B-O-M signal is not detected at the beginning of a bit stream, the transmission cannot be initiated. When noise is present or other interference interrupts the transmission, the system detects the next received bit in the data stream as a new transmission. It looks for a sync or B-O-M signal, and since none is present, the reception is not initiated, even though the sending station continues to send reliable data.
Therefore, it would be desirable if the receiving station had the capability to collect data both before and after an interruption point in order permit recovery of a "lost" transmission to permit recovery after interruption and "look back in time" for valid transmitted data. This would permit "reconstruction" of a lost transmission.